Rapid Cell Density Measurement Technique to Help Predict Immunotherapy Response

As cells go through processes such as proliferation, differentiation, or cell death, they may gain or lose water and other molecules, which results in changes in their density. While tracking these subtle changes in the physical state of cells is challenging, especially at a single-cell resolution, a team of researchers has developed a way to quickly and accurately measure cell density, capturing data from up to 30,000 cells in just one hour. This method also shows that changes in cell density can be used to make useful predictions, such as determining if immune cells like T cells have been activated to fight tumors or if tumor cells are responsive to a particular drug.

As cells change states, their molecular components, including lipids, proteins, and nucleic acids, can become more densely packed or more dispersed. Measuring the density of a cell provides an indirect way to observe this molecular crowding. The new technique for measuring cell density, developed by researchers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA), builds on previous advancements in cell and particle measurement technologies. In 2007, the team created a microfluidic device known as a suspended microchannel resonator (SMR), which consists of a tiny silicon cantilever vibrating at a specific frequency. As a cell passes through the channel, the frequency of the vibration changes slightly, and the magnitude of this change can be used to calculate the cell’s mass.

In 2011, the researchers modified the technique to enable the measurement of cell density. To do this, the cells are passed through the device twice, each time suspended in a different liquid with varying densities. The buoyant mass of a cell—its mass as it floats in the liquid—is determined by its absolute mass and volume. By measuring two different buoyant masses, the researchers can calculate the cell’s mass, volume, and density. While this method works effectively, it requires switching fluids and running cells through each one, which is time-consuming and limits the number of cells that can be measured. To speed up the process, the team combined the SMR device with a fluorescent microscope, allowing for rapid volume measurements. The microscope is placed at the entrance of the resonator, and the cells flow through while suspended in a fluorescent dye that is not absorbed by the cells. As the cells pass the microscope, the dip in the fluorescent signal provides the volume measurement.

After measuring the cell volume, the cells flow into the resonator, where their mass is measured. This combination of techniques allows for quick calculation of cell density and enables the measurement of up to 30,000 cells in just one hour. Using this new method, the researchers tracked the changes in the density of T cells after they were activated by signaling molecules. As T cells transition from a resting state to an active state, they accumulate new molecules and water. The team found that the density of the cells dropped from an average of 1.08 grams per milliliter to 1.06 grams per milliliter between their pre-activation state and the first day of activation, indicating that the cells became less crowded as they gained water more quickly than other molecules. The researchers are now exploring how the SMR mass measurements can be used to predict whether T cells in individual cancer patients will respond to drugs designed to trigger a strong anti-tumor immune response. Preliminary studies suggest that using both mass and density measurements together provides a more accurate prediction than using either one alone.

Another promising application of this technique is in predicting how tumor cells will respond to various cancer treatments. In previous studies, the team demonstrated that changes in cell mass after treatment could predict whether tumor cells were undergoing drug-induced cell death. In this new study, published in Nature Biomedical Engineering, the researchers found that changes in cell density could also provide similar insights. When testing two different drugs on pancreatic cancer cells—one that the cells were sensitive to and one they were resistant to—they found that the density changes after treatment accurately reflected the cells' responses. Moving forward, the team plans to use measurements of cell mass and density to assess the fitness of cells used in the production of complex proteins, such as therapeutic antibodies.

“These data are pointing to the notion that cell density is an interesting biomarker that is changing during T-cell activation and may have functional relevance to how well the T cells could proliferate,” said Weida (Richard) Wu, MIT research scientist and the paper’s lead author.

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